The small subunit of T4 terminase, gpl6, in association with the large subunit, gp 17, is involved in the initiation of concatemer processing in packaging of the DNA. The monomer is 19 k') but is an apparently homogeneous oligomer on gels (about a 10 to 20mer). In conventional EM, the oligomers appear as rings. Mass measurements were made on freeze-dried samples and negatively stained samples were examined in the STEM. The results indicate that gp 16 forms specific oligomers, rings and side-by-side double rings. The single ring contains about eight monomers, and the rings have a diameter of about 8 nm with a central hole of about 2 nm.